Individual running with energy (3)
I believe the answer is:
one
'individual' is the definition.
(an individual is one person)
'running with energy' is the wordplay.
'running' becomes 'on' (eg when a machine is running it is on).
'with' is a charade indicator (letters next to each other).
'energy' becomes 'e' (physics symbol as in E=mc2).
'on'+'e'='ONE'
(Other definitions for one that I've seen before include "Sole; identical" , "Lowest cardinal number" , "both halves" , "This is singular" , "Single and undivided" .)